Login the web GUI, go into “FXS -> SIP-->advanced” page, Enable SIP 100REL, click Save button and restart the device.

Note: The parameter PRACK in the new version is changed to enable SIP 100REL.

Test Result

PRACK is generally a confirmation of the 183 call in progress / 180 ringing received.

The INVITE initiated by the UAC contains Supported: 100 rel. The UAS also supports this extension and has a Require: 100rel in the 183 response, indicating that there should be a PRACK response to all 1xx responses other than 100 in the next session.